The present invention relates to cementing operations and, more particularly, in certain embodiments, to calcium phosphate cement compositions that utilize pumice and/or perlite as a replacement for fly ash.
Cement compositions may be used in a variety of subterranean applications. For example, in subterranean well construction, a pipe string (e.g., casing, liners, expandable tubulars, etc.) may be run into a well bore and cemented in place. The process of cementing the pipe string in place is commonly referred to as “primary cementing.” In a typical primary cementing method, a cement composition may be pumped into an annulus between the walls of the well bore and the exterior surface of the pipe string disposed therein. The cement composition may set in the annular space, thereby forming an annular sheath of hardened, substantially impermeable cement (i.e., a cement sheath) that may support and position the pipe string in the well bore and may bond the exterior surface of the pipe string to the subterranean formation. Among other things, the cement sheath surrounding the pipe string functions to prevent the migration of fluids in the annulus, as well as protecting the pipe string from corrosion. Cement compositions also may be used in remedial cementing methods, for example, to seal cracks or holes in pipe strings or cement sheaths, to seal highly permeable formation zones or fractures, to place a cement plug, and the like.
Portland cement is commonly used in subterranean cementing applications. Drawbacks may exist to using Portland cements in certain applications, however, because such cements are prone to corrosive attacks by carbonic acid (H2CO3). Carbonic acid may be naturally present in a subterranean formation, or it may be produced in the formation by the reaction of subterranean water and carbon dioxide (CO2), when the latter has been injected into the formation, e.g., as in a CO2-enhanced recovery operation. Carbonic acid is believed to react with calcium hydroxide that is present in Portland cement, which reaction may cause the cement to become a soft amorphous gel. This is problematic because, inter alia, it may increase the permeability of the cement. As a result, chloride and hydrogen sulfide ions, which may be present in the subterranean formation, may penetrate the cement sheath and adversely affect, or react with, the casing. The degradation of the cement can cause, inter alia, loss of support for the casing and undesirable interzonal communication of fluids.
It has heretofore been discovered that a set cement material known as calcium phosphate cement formed by an acid-base reaction between calcium aluminate and a phosphate-containing solution can have, for example, high strength, low permeability and excellent carbon dioxide resistance when cured in hydrothermal environments. Compositions containing calcium aluminate and a phosphate-containing solution that react to form calcium phosphate cements may generally be referred to as calcium phosphate cement compositions. Fly ash is often included in the calcium phosphate cement compositions as it is believed that the fly ash reacts with components in the composition to form calcium aluminosilicates, which are resistant to carbonic acid corrosion. However, because fly ash is a waste material there may be drawbacks to its use in the cement compositions. For example, the composition of the fly ash may vary depending upon its source, making it more difficult to design compositions with the fly ash as the variability in composition can impact properties of the cement composition, including its thickening time and pumpability, among others. In addition, supply issues have been encountered with fly ash making its availability as a cement additive unpredictable in some instances.